Voltage detection circuits, and particularly CMOS voltage detection circuits, that have the ability to provide an output signal once an input voltage has passed a certain voltage threshold are known in the art. Voltage detection circuits are important building blocks used in the design of many types of analog and mixed analog/digital integrated circuits. Current comparators are widely used, for example, in POR (“Power On Reset”) circuits.
A POR circuit typically detects when the supply voltage for an integrated circuit reaches a predetermined acceptable level, and sends an output signal to reset internal memory elements on the integrated circuit. Voltage detection circuits, which compare the supply voltage against a known reference voltage, are often included in POR circuits to determine when a safe operating voltage is reached. The difficulty in designing POR circuits is threefold: first, the reference voltage must be generated from the same supply voltage that is ramped up to a final supply voltage; second, the reference voltage must not vary excessively over process, voltage and temperature (“PVT”) conditions, or else the POR circuit sends out a signal prematurely, or not at all; and third, in the case of low voltage processes, the reference voltage used must be small (in the one volt range).
Typically, diode clamp and band-gap circuits have been used to generate a low voltage reference. However, because the voltage across a diode is very sensitive to temperature, diode clamp circuits cannot meet the strict voltage detection tolerances needed in some applications. Additionally, bandgap circuit, which provide a temperature-insensitive output voltage, usually contain too many components to be cost-effective when used in an integrated circuit application.
Referring now to FIG. 1, a typical diode clamp voltage detect circuit 10 is shown for generating an output voltage as the VDD voltage supply ramps up to a final supply voltage level. The VOUT voltage in detect circuit 10 tracks the VDD voltage, but at a lower voltage level determined by the ratio of the value of resistors R1 and R2, as well as the voltage drop across diode-connected transistor P1. Circuit 10 is not a good candidate for use in a POR circuit due to the temperature sensitivity of drain-to-source voltage drop across transistor P1.
Referring now to FIG. 2, another typical diode clamp voltage detect circuit 20 is shown for generating an output voltage as the VDD voltage supply ramps up to a final supply voltage level. The VOUT voltage in detect circuit 10 initially tracks the VDD voltage, but quickly levels out to a reference voltage level determined by the ratio of the value of resistors R1 and R2, as well as the voltage drop across diode-connected transistor N1. Circuit 20 is also not a good candidate for use in a POR circuit due to the temperature sensitivity of drain-to-source voltage drop across transistor N1.
Referring now to FIG. 3, a bandgap circuit is shown for generating an output voltage as the VDD voltage supply ramps up to a final supply voltage level. The VOUT voltage in detect circuit 30 also initially tracks the VDD voltage, but quickly levels out to a reference voltage level determined a bandgap circuit including a first transistor in parallel with the combination of a second transistor in series with a resistor, as well as a feedback circuit (not shown in FIG. 3). While circuit 30 is a good candidate for use in a POR circuit from a performance standpoint, it is not a good candidate for low voltage operation (bandgap of silicon, which is typically used is 1.1 volts) or from a cost standpoint due to the number of devices that must be used in the circuit.
What is desired, therefore, is a voltage detection circuit for providing a temperature-insensitive output voltage, but is realized with a design that can be economically implemented in an integrated circuit.